Lubuntu as an official derivative of Ubuntu

The four main desktop environments are GNOME, KDE, XFCE and LXDE. While Kubuntu and Xubuntu are considered official derivatives of Ubuntu, Lubuntu is not. Why is that so?

Update: Lubuntu has gained official status in 11.10.

Solution 1:

At the spring 2011 Ubuntu Developer Summit, it was decided that Lubuntu was on track to become an official derivative with the release of 11.10. More information on the decision can be found in this 16 May 2011 email by Lubuntu developer Julien Lavergne.

As stated on their website, Lubuntu still aims to become a recognized derivative:

The ultimate goal of the lubuntu project is to earn official endorsement from Canonical.

Solution 2:

Lubuntu is now an official derivative. :)